1. Field of the Invention
The present invention relates to a method for manufacturing magnetic paint, a method for manufacturing non-magnetic paint and a magnetic recording medium. In particular, the present invention relates to a method for manufacturing magnetic paint and a method for manufacturing non-magnetic paint, which can produce a magnetic recording medium having excellent surface smoothness by improving the dispersion condition during a dispersion process using a medium dispersion device, and to a magnetic recording medium using the magnetic paint and the non-magnetic paint (hereafter may be collectively, briefly referred to as “paint”) produced by the respective methods.
2. Description of the Related Art
In general, magnetic paint or non-magnetic paint for manufacturing a magnetic recording medium is manufactured through, for example, steps of supplying a magnetic paint composition or a non-magnetic paint composition composed of a magnetic powder or a non-magnetic powder, a binder, an organic solvent and other necessary components into, e.g. a medium dispersion type mill, in which dispersion media, e.g. glass beads, are filled in a mixing tank, and performing forced-agitation together with the dispersion media by using an agitator built in the mixing tank.
On the other hand, regarding magnetic recording media used for videos, audio equipment, computers or the like, recording densities have been more and more increased in recent years. Consequently, there is a trend toward reduction of the minimum recording unit by reducing recording wavelengths, reducing track widths and reducing recording medium thicknesses. In order to match them, a ferromagnetic metal powder composed of fine particles having large magnetic energy is now going into use as the magnetic powder. However, the cohesion of individual particles in the magnetic powder is enhanced with a decrease in particle size or an increase in magnetic energy. As a result, in a method for manufacturing magnetic paint by using glass beads as dispersion media, there are problems in that dispersibility and surface smoothness required for achieving a high playback output of short wavelength recording and an excellent S/N ratio cannot be sufficiently attained.
In order to overcome the problems, methods using ceramic beads, e.g. zircon beads and zirconia beads, having specific gravities larger than that of glass beads as dispersion media are proposed in Japanese Unexamined Patent Application Publication No. 60-211637, Japanese Unexamined Patent Application Publication No. 64-57422, Japanese Unexamined Patent Application Publication No. 1-290122 and the like.
However, when ceramic beads having a relatively large specific gravity, in particular zirconia beads (specific gravity 6 g/cc) are used as dispersion media, there are problems in that the magnetic powder is damaged, and the electromagnetic transducing characteristic is degraded because the specific gravity is too large. In order to avoid this problem, some methods are considered in which the viscosity of the mixed solution to be subjected to a dispersion process is increased, or the agitator of the dispersion device is operated at a circumferential speed lower than that in a conventional condition. However, these methods cause problems of the applicability and the dispersibility of the paint.
On the other hand, the applicant of the present invention proposed a technology relating to a method for manufacturing magnetic paint, in which a predetermined dispersion media were used during preparation of paint, and a dispersion process performed under a predetermined condition, and to a magnetic recording medium in Japanese Unexamined Patent Application Publication No. 2001-81406. According to this technology, magnetic paint having various excellent medium characteristics, and by extension, a magnetic recording medium can be produced without causing the above-mentioned problems.
In order to achieve reduction in thickness of a magnetic recording medium in response to the above-mentioned requirement for an increase in recording density, it is important that the magnetic layer or the non-magnetic layer is formed from a thin film. However, in the technology described in the above-mentioned publication, the condition of the formation using a thin film is not sufficiently discussed. Since the surface property of the magnetic recording medium directly exerts an effect on the medium characteristics, the surface smoothness of the magnetic recording medium must be increased in order to attain excellent medium characteristics. Consequently, regarding the manufacture of the magnetic recording medium including a non-magnetic layer and a magnetic layer, each having a small thickness, a technology for achieving excellent dispersibility and for realizing a magnetic recording medium having excellent surface smoothness has been required.